来自KMTNet超新星计划的快速下降的无主Ia型超新星KSP-OT-201509 b：过渡性质和对$^{56}$ Ni分布和祖先类型的限制
Rapidly-Declining Hostless Type Ia Supernova KSP-OT-201509b from the KMTNet Supernova Program: Transitional Nature and Constraint on $^{56}$Ni Distribution and Progenitor Type
摘要：
我们报告了KMTNet超新星计划中一颗快速下降的亚克拉塞卡Ia型超新星KSP-OT-201509 b（= AT 2015 cx）的早期发现和多色（$BVI$）高节奏光变曲线分析。KSP-OT-201509 b（= AT 2015 cx）的Phillips参数和颜色拉伸参数分别为$Δ$ $M_{B,15}$ $\simeq$ 1.62 mag和$s_{BV}$ $\simeq$ 0.54，推断红移为0.072。这些，连同其他测量参数（如二级$I$带峰的强度，颜色和光度），确定源是一个过渡性质的快速下降的Ia型，比91 bg更接近分支正常。它的早期光变曲线演化和热光度与由放射性衰变提供动力的同源膨胀喷出物和Ia型SN爆炸一致，其中合成的$^{56}$ Ni质量为0.32 $\pm$ 0.01 $M_{\odot}$，喷出物质量为0.84 $\pm$ 0.12 $M_{\odot}$，喷出物动能为（0.61 $\pm$ 0.14）$\times$ 10 $^{51}$尔格。虽然它的$B-V$和$V-I$颜色的演变与其他超新星中发现的$I$波段光变曲线的变化基本同步，但我们也发现$V-I$在峰值后10天之前就存在早期的红色演变。辐射热光变曲线的源是兼容的分层$^{56}$镍分布扩展到浅层的爆炸祖。观测到的光变曲线和从喷出物-伴星相互作用预测的光变曲线之间的比较明显不利于在大的分离距离处充满洛希叶的伴星，从而支持其起源的双简并方案。在我们的深度堆栈图像中缺乏任何明显的宿主星系，达到了$\sim$ 28 $\rm mag\;arcsec^{-2}$的灵敏度极限，这使得KSP-OT-201509 b成为一颗没有宿主的Ia型超新星，并为超新星宿主星系环境提供了新的见解。
英文摘要：
We report the early discovery and multi-color ($BVI$) high-cadence light curve analyses of a rapidly-declining sub-Chandrasekhar Type Ia supernova KSP-OT-201509b (= AT2015cx) from the KMTNet Supernova Program. The Phillips parameter and color stretch parameter of KSP-OT-201509b (= AT2015cx) are $Δ$$M_{B,15}$ $\simeq$ 1.62 mag and $s_{BV}$ $\simeq$ 0.54, respectively, at an inferred redshift of 0.072. These, together with other measured parameters (such as the strength of the secondary $I$-band peak, colors and luminosity), identify the source to be a rapidly-declining Type Ia of transitional nature that is closer to Branch Normal than 91bg-like. Its early light curve evolution and bolometric luminosity are consistent with those of homologously expanding ejecta powered by radioactive decay and a Type Ia SN explosion with 0.32 $\pm$ 0.01 $M_{\odot}$ of synthesized $^{56}$Ni mass, 0.84 $\pm$ 0.12 $M_{\odot}$ of ejecta mass and (0.61 $\pm$ 0.14) $\times$ 10$^{51}$ erg of ejecta kinetic energy. While its $B-V$ and $V-I$ colors evolve largely synchronously with the changes in the $I$-band light curve as found in other supernovae, we also find the presence of an early redward evolution in $V-I$ prior to --10 days since peak. The bolometric light curve of the source is compatible with a stratified $^{56}$Ni distribution extended to shallow layers of the exploding progenitor. Comparisons between the observed light curves and those predicted from ejecta-companion interactions clearly disfavor Roche Lobe-filling companion stars at large separation distances, thus supporting a double-degenerate scenario for its origin. The lack of any apparent host galaxy in our deep stack images reaching a sensitivity limit of $\sim$ 28 $\rm mag\;arcsec^{-2}$ makes KSP-OT-201509b a hostless Type Ia supernova and offers new insights into supernova host galaxy environments.
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